Radiation-curable silicone coatings for release of pressure-sensitive adhesives are known. For example, ultraviolet-curable silicone compositions containing acrylic functional silicone polymers are taught in U.S. Pat. Nos. 4,201,808 and 4,678,846. In addition, silicone-release compositions containing ultraviolet curable epoxy-functional silicones are taught in U.S. Pat. No. 4,421,904. Each of these systems, as well as others based upon similar or related chemistries, produce release substrates exhibiting specific and useable ranges of release performance.
However, there are other significant applications, such as that of industrial labeling operations, for which the release performance ranges of such systems are not as ideal. Often, the release performance of these radiation-curable silicone compositions are found to depend upon the type of pressure-sensitive adhesive being used in the construction of the label. For that reason it is sometimes necessary to be able to raise the release force, i.e., tighten the release.
Extensive efforts have been made to achieve the objective of extending the release performance range of these systems to higher values. For example, U.S. Pat. No. 4,568,566 discusses the use of acrylic functional silicone resins in UV-curable acrylic functional polymers. In addition, numerous patents have been fried for those systems employing UV-curable epoxy functional polymers. U.S. Pat. Nos. 4,547,431 and 5,217,805 discuss the use of polyfunctional epoxy monomers as control release additives (CRAs) and U.S. Pat. No. 4,952,657 teaches modification of UV-curable epoxy-functional silicones with pendant attached phenolic groups. Several other patents focus on the use of various forms of functionalized MQ resins having methyl and/or alkenyl (U.S. Pat. No. 5,198,476), or epoxy (U.S. Pat. Nos. 5,279,860; 5,310,601; and 5,360,833) groups in the UV-curable epoxy-functional silicone polymer.
While each of these approaches have provided the capability to control upward the release force of the acrylic, epoxy, and vinylether functionalized radiation-curable silicone systems noted above, none of these efforts have been successful in lowering the release force below that which is the normal minimum for each of these radiation-curable silicone compositions. And often, that minimum release force is above what has come to be known as the premium release level.
In non-radiation cure silicone-release coating compositions, premium release in such coatings is characteristic of the base silicone polymer. Thermal cure versions of these polymers are generally standard polydimethylsiloxanes which have been terminated with either vinyl or silanic hydrogen reactive groups, or they are copolymers of polydimethyl, methylvinylsiloxane. These copolymers can also be terminated with vinyl reactive groups as in the case of the standard polydimethylsiloxanes. The natural premium release behavior of these polymers make them potential candidates for blends into radiation-curable silicones.